It is known to fit auxiliary brakes on a vehicle that is equipped with an engine and drive wheels, with transmission components incorporated therebetween, and which components include a main transmission with which a driver of the vehicle can select different gears for forward and reverse driving. Auxiliary brakes are utilized primarily in heavier vehicles with a primary aim of saving the service brakes of the vehicle, especially when negotiating long downhill gradients when braking is desirable to maintain fairly constant speed. Through the use of the auxiliary brakes, the service brakes can be kept fresh so that when the vehicle really needs to sharply decelerate, maximum brake force is available from the service brakes. In typical configurations, the service brakes have much more powerful braking capabilities than do the auxiliary brakes, and at least partly because the service brakes are normally fitted on all the wheels of the vehicle. In contrast, the auxiliary brakes normally act only upon the drive wheels.
It is further known to differentiate between so-called primary and secondary auxiliary brakes in a vehicle. Primary and secondary alludes to the positioning of such auxiliary brakes in front of, or behind the main transmission of the vehicle. Examples of primary auxiliary brakes are ISG (Integrated Starter & Generator) and retarders. A retarder is usually of the hydrodynamic retarder- or electromagnetic retarder-type. These type of brakes are normally disposed between the engine and the main transmission. A primary auxiliary brake can also be constituted by various types of engine brakes; for example, compression brakes, exhaust brakes or brakes that utilize the friction of the engine to slow the vehicle. The braking energy in compression and exhaust brakes is converted mainly into heat, which in large part is dissipated via the engine cooling system. It should be noted, however, that a substantial part (approximately forty percent of the braking energy) is carried with the exhaust gases of the vehicle out through the exhaust system of the vehicle. Utilization of the friction of the engine for braking purposes can be regulated by the injection of regulated quantities of fuel into the engine in amounts that permit the output torque from the engine to be, for example, zero. Another option is to uncouple the engine from the other driveline using a clutch that is disposed between the engine and the transmission. In this context, the terminology, “driveline” describes the engine of the vehicle, the drive wheels, and the transmission components coupling the engine to the drive wheels. Other controllable assemblies coupled to the engine, and which impact on the brake force from the engine, can include an engine cooler fan, an air-conditioning unit, a compressed-air compressor, and other ancillary assemblies that may be coupled to the engine.
A secondary auxiliary brake, which is disposed somewhere behind the main transmission of the vehicle, is usually constituted by a retarder of hydrodynamic or electromagnetic type.
When a vehicle is equipped with powerful auxiliary brakes, for example both primary and secondary auxiliary brakes, or a plurality of just the primary type, there is a high risk that the combined brake force is so great that in certain situations some transmission components are exposed to stresses in excess of their maximum torque capacity.
From U.S. Pat. No. 5,921,883, it is known to control brake torque from a compression brake as a function of the speed or engaged gear of the vehicle, with a view to not exceeding the torque capacity of a transmission component upon compression-braking of the vehicle. In an illustrated embodiment, the weight of the vehicle and the gradient of the roadway are taken into account. In the control system of the '883 patent, the torque capacity in case of compression-braking for the weakest transmission component and the characteristics of the compression brake are stored; for example, in a table showing how much brake torque the compression brake produces at a certain revolutionary speed, setting, or the like.
In a situation in which compression braking is imminent, the control system makes a comparison only if the requested brake torque exceeds the torque capacity of the system. Then, if the requested brake torque exceeds the torque capacity of any of the interconnecting components, the control system chooses a value which lies below the maximum torque capacity. Control systems of this nature conduct the comparison only against a table stored in the control system, the values of which are derived in a laboratory environment through simulations or dynamometer tests. The values are often compromises since it would be too expensive to take into account any specific peculiarities of individual vehicles and specific situations. Examples of such vehicle characteristics and situations include when the vehicle is cold and lubricating oils in engine and transmission are viscous and brake differently than at normal running temperature or when various parts in the drive line have worn to an extent that their capacities are reduced, or reducing. Furthermore, there is no feedback utilized in the regulation of the estimation by which the values are verified.
In U.S. Pat. No. 5,921,883, only a single auxiliary compression-type brake is controlled. Nor does the system of the '883 patent take any account of whether the brake force from the auxiliary brake is too high for the friction between the roadway and the drive wheels; that is to say, a comparison to when the vehicle will start to skid.
DE 4420116 (corresponding to WO9533631A1) shows a procedure for controlling auxiliary brake torque in a vehicle. The auxiliary brake is constituted by an engine brake and a retarder. When a temperature sensor senses that the water temperature in the cooling system of the vehicle is too high, the brake action of the hydrodynamic retarder is moderated with a purpose of avoiding overheating.
An auxiliary brake of the hydrodynamic retarder-type usually includes an impeller (rotor) and a turbine wheel (stator). The rotor is fixedly coupled to, for example, the cardan shaft of the vehicle and rotates therewith. The stator is fixedly disposed in a retarder housing in which both the rotor and the stator are enclosed. The retarder housing is connected to an oil reservoir. When oil is squeezed into the retarder housing, it is set in motion by the rotor which squeezes the oil against the stator. Since the stator cannot rotate, the flow of oil is retarded, whereby the rotor and the whole of the vehicle are braked. The brake torque is regulated by the quantity of oil in the retarder housing. The heat which is generated when the oil slows the rotor is usually dissipated via a heat exchanger coupled to the engine cooling system. This means that the retarder requires more cooling capacity from the engine cooling system compared with, for example, the above-stated compression or exhaust brakes in which a large part of the braking energy is outwardly dissipated directly through the exhaust system. The maximum braking capacity of a retarder can usually be utilized only for relatively short periods, owing to the inadequate capacity of the cooling system.
An auxiliary brake of the electromagnetic retarder type usually comprises a stator in the form of electromagnets and a rotor in the form of soft-iron plates. The rotor is coupled to, for example, the cardan shaft of the vehicle and the stator is fixedly mounted in the vehicle. When current is connected to the electromagnets, a brake torque is generated upon the rotor when it rotates. The braking energy is converted into heat owing to the eddy currents formed in the soft-iron plate. In case of lengthy braking, the rotor is heated up so much that the formation of eddy currents is inhibited. This can lead to a reduction in braking capacity, at least in the case of lengthy use and maximum utilization of the capacity of the retarder, and even possibly to the total loss, in principle, of the braking capacity. Also, such electromagnetic retarders are usually cooled by ambient air.
Vehicles equipped with more than one auxiliary brake often have more brake force at their disposal and therefore run a greater risk of exceeding the torque capacity of some transmission components in case of auxiliary braking.
Vehicles equipped with at least two purely primary auxiliary brakes (in which one is typically a hydrodynamic retarder and the other is an auxiliary brake, for example, a compression brake) often enter into situations in which full power from both the auxiliary brakes is unnecessary. An example of such a situation is the case in which lengthy auxiliary braking is employed and the retarder utilizes an unnecessarily large amount of the capacity of the cooling system because the braking energy of the retarder has to be dissipated using the vehicle cooling system. After a relatively short time period, auxiliary braking has to be halted because of overheating of the vehicle's cooling system.
In similar configurations, but where one of the two auxiliary brakes is an electromagnetic retarder, the retarder can become so heated up that there is a risk of a reduction or total loss in braking capacity from the retarder. This can be a problem if the driver, under such conditions, requests maximum auxiliary brake force from both the auxiliary brakes. The available brake force will not be sufficient, and this can lead to the service brakes of the vehicle having to be used instead.
There is therefore a need for a method and (arrangement) device for reciprocally controlling or regulating the auxiliary brakes of a vehicle, in which the type of auxiliary brake and the torque capacity of the transmission components are taken into account. This is a main object of the invention(s) disclosed hereinbelow.